The present invention relates to a plant for transmitting electric power through High Voltage Direct Current (HVDC) comprising two converter stations interconnected by a bipolar direct voltage network and each connected to an alternating voltage network for feeding electric power from one of said alternating voltage networks to the other, each converter station having a Voltage Source Converter having at least one phase leg connecting to opposite poles of the direct voltage side of the converter and comprising a series connection of switching cells, each said switching cell having on one hand at least two semiconductor assemblies connected in series and having each a semiconductor device of turn-off type and a free-wheeling diode connected in parallel therewith and on the other at least one energy storing capacitor, a phase output of the converter being configured to be connected to an alternating voltage side of the converter being formed between two switching cells along said series connection of switching cells, each said switching cell being configured to obtain two switching states by control of said semiconductor devices of each switching cell, namely a first switching state and a second switching state, in which the voltage across said at least one energy storing capacitor and a zero voltage, respectively, is applied across the terminals of the switching cell, for obtaining a determined alternating voltage on said phase output.
It is possible that one of the converter stations always functions as rectifier, i.e. electric power is fed from the alternating voltage network connected to this station and to the other converter station operating as inverter and through this to the alternating voltage network connected to the converter station last mentioned. This would be the case if the alternating voltage network connected to the converter station first mentioned is a part of a plant for generating electric power, such as a wind power park. However, it is also conceivable that the feeding of electric power between the two converter stations may change, so that the converter stations may change from an operation as rectifier to inverter and conversely.
The converters in said stations may have any number of said phase legs, but they have normally three such phase legs for having a three phase alternating voltage on the alternating voltage side thereof.
A Voltage Source Converter of this type may be used in all kinds of situations, in which direct voltage is to be converted into alternating voltage and conversely, in which examples of such uses are in stations of HVDC-plants (High Voltage Direct Current), in which direct voltage is normally converted into a three-phase alternating voltage or conversely, or in so-called back-to-back stations in which alternating voltage is firstly converted into direct voltage and this is then converted into alternating voltage, as well as in SVCs (Static Var Compensator), in which the direct voltage side consists of capacitors hanging freely.
A Voltage Source Converter of this type is known through for example DE 101 03 031 A1 and WO 2007/023064 A1 and is as disclosed there normally called a multi-cell converter or M2LC. Reference is made to these publications for the functioning of a converter of this type. Said switching cells of the converter may have other appearances than those shown in said publications, and it is for instance possible that each switching cell has more than one said energy storing capacitor, as long as it is possible to control the switching cell to be switched between the two states mentioned in the introduction.
The present invention is directed to such Voltage Source Converters configured to transmit high powers. When such a Voltage Source Converter is used to transmit high powers this also means that high voltages are handled, and the voltage of the direct voltage side of the converter is determined by the voltages across said energy storing capacitors of the switching cells. This means that a comparatively high number of such switching cells are to be connected in series for a high number of semiconductor devices, i.e. said semiconductor assemblies are to be connected in series in each said switching cell, and a Voltage Source Converter of this type is particularly interesting when the number of the switching cells in said phase leg is comparatively high. A high number of such switching cells connected in series means that it will be possible to control these switching cells to change between said first and second switching state and by that already at said phase output obtain an alternating voltage being very close to a sinusoidal voltage. This may be obtained already by means of substantially lower switching frequencies than typically used in known Voltage Source Converters of the type shown in FIG. 1 in DE 101 03 031 A1 having switching cells with at least one semiconductor device of turn-off type and at least one free-wheeling diode connected in anti-parallel therewith. This makes it possible to obtain substantially lower losses and also considerably reduces problems of filtering and harmonic currents and radio interferences, so that equipment therefor may be less costly.
Accordingly, a plant for transmitting electric power through HVDC having converter stations with such Voltage Source Converters of M2LC-type has a number of advantages.